The present invention relates generally to methods for fabricating porous materials, and more particularly to such methods using a reverse fabrication technique, utilizing a three-dimensional negative replica formed from a porogen material. The resulting variety of inventive porous materials may be used for many different applications such as tissue engineering scaffolds, cell culture matrices, controlled release matrices, wound dressings, separation membranes, column fillers of chromatography, filters, packaging and insulating materials, and so forth.
Engineering tissues and organs with mammalian cells and a scaffolding material is a new approach compared to the use of harvested tissues and organs. See Langer, R. S. and J. P. Vacanti, xe2x80x9cTissue engineering: the challenges ahead,xe2x80x9d Scientific American 280(4), 86 (1999). In the tissue engineering approach, the scaffold plays a pivotal role in cell seeding, proliferation, and new tissue formation in three dimensions. See Langer, R. and J. Vacanti, xe2x80x9cTissue engineering,xe2x80x9d Science 260(5110), 920-926 (1993); Hubbell, J. A., xe2x80x9cBiomaterials in Tissue Engineering,xe2x80x9d Bio/Technology 13, 565 (1995); and Saltzman, W. M., xe2x80x9cCell Interactions with Polymers,xe2x80x9d Principles of Tissue Engineering, R. Lanza, R. Langer and W. Chick, Editors, (1997) Academic Press, R. G. Landes Company, Austin, Tex., 225. Biodegradable polymers have been attractive candidates for scaffolding materials because they degrade as the new tissues are formed, eventually leaving nothing foreign to the body. See Ma, P. X. and R. Langer, xe2x80x9cDegradation, structure and properties of fibrous nonwoven poly(glycolic acid) scaffolds for tissue engineering,xe2x80x9d Polymers in Medicine and Pharmacy, A. G. Mikos, K. W. Leong, M. L. Radomsky, J. A. Tamada and M. J. Yaszemski, Editors, (1995) MRS, Pittsburgh, 99-104. A few techniques such as salt-leaching (see Mikos, A. G., A. J. Thorsen, L. A. Czerwonka, Y. Bao, R. Langer, D. N. Winslow and J. P. Vacanti, xe2x80x9cPreparation and characterization of poly(l-lactic acid) foams,xe2x80x9d Polymer 35(5), 1068-1077 (1994); and Ma, P. X. and R. Langer, xe2x80x9cFabrication of biodegradable polymer foams for cell transplantation and tissue engineering,xe2x80x9d Tissue Engineering Methods and Protocols, M. Yarmush and J. Morgan, Editors, (1998) Humana Press Inc., Totowa, N.J.), fibrous fabric processing, 3-D printing (see Park, A., B. Wu and L. G. Griffith, xe2x80x9cIntegration of surface modification and 3D fabrication techniques to prepare patterned poly(L-lactide) substrates allowing regionally selective cell adhesion,xe2x80x9d Journal of Biomaterials Science Polymer Edition 9(2), 89-110 (1998)), and phase-separation (see Zhang, R. and P. X. Ma, xe2x80x9cPoly (alpha-hydroxy acids)/hydroxyapatite porous composites for bone tissue engineering: 1. Preparation and morphology,xe2x80x9d Journal of Biomedical Materials Research 44(4), 446-455 (1999); Zhang, R. and P. X. Ma, xe2x80x9cPorous poly(L-lactic acid)/apatite composites created by biomimetic process,xe2x80x9d Journal of Biomedical Materials Research 45(4), 285-293 (1999); Ma, P. X. and R. Zhang, xe2x80x9cSynthetic nano-scale fibrous extracellular matrix,xe2x80x9d Journal of Biomedical Materials Research 46(1):60-72 (May 3, 1999); and Lo, H., S. Kadiyala, S. E. Guggino and K. W. Leong, xe2x80x9cPoly(L-lactic acid) foams with cell seeding and controlled-release capacity,xe2x80x9d J Biomed Mater Res 30(4), 475-484 (1996)) have been developed to generate highly porous polymer scaffolds for tissue engineering.
These scaffolds have shown great promise in the research of engineering a variety of tissues. See, for example, Vacanti, C. A. and L. J. Bonassar, xe2x80x9cAn overview of tissue engineered bone,xe2x80x9d Clinical Orthopaedics and Related Research (367 Suppl), S375 (1999); Freed, L. E., R. Langer, I. Martin, N. R. Pellis and G. Vunjak-Novakovic, xe2x80x9cTissue engineering of cartilage in space,xe2x80x9d Proceedings of the National Academy of Sciences of the United States of America 94(25), 13885-13890 (1997); Ma, P. X., B. Schloo, D. Mooney and R. Langer, xe2x80x9cDevelopment of biomechanical properties and morphogenesis of in vitro tissue engineered cartilage,xe2x80x9d J Biomed Mater Res 29(12), 1587-1595 (1995); Ma, P. X. and R. Langer, xe2x80x9cMorphology and mechanical function of long-term in vitro engineered cartilage,xe2x80x9d Journal of Biomedical Materials Research 44(2), 217-221 (1999); Cao, Y., J. Vacanti, X. Ma, K. Paige, J. Upton, Z. Chowanski, B. Schloo, R. Langer and C. Vacanti, xe2x80x9cGeneration of neo-tendon using synthetic polymers seeded with tenocytes,xe2x80x9d Transplant Proc 26(6), 3390-3392 (1994); Ibarra, C., C. Jannetta, C. A. Vacanti, Y. Cao, T. H. Kim, J. Upton and J. P. Vacanti, xe2x80x9cTissue engineered meniscus: a potential new alternative to allogeneic meniscus transplantation,xe2x80x9d Transplantation Proceedings 29(1-2), 986 (1997); Cusick, R. A., H. Lee, K. Sano, J. M. Pollok, H. Utsunomiya, P. X. Ma, R. Langer and J. P. Vacanti, xe2x80x9cThe effect of donor and recipient age on engraftment of tissue engineered liver,xe2x80x9d Journal of Pediatric Surgery 32(2), 357 (1997); Shinoka, T., P. X. Ma, D. Shum-Tim, C. K. Breuer, R. A. Cusick, G. Zund, R. Langer, J. P. Vacanti and J. E. Mayer, Jr., xe2x80x9cTissue-engineered heart valves, Autologous valve leaflet replacement study in a lamb model,xe2x80x9d Circulation 94(9 Suppl), 11-164-168 (1996); Shinoka, T., D. Shum-Tim, P. X. Ma, R. E. Tanel, N. Isogai, R. Langer, J. P. Vacanti and J. E. Mayer, Jr., xe2x80x9cCreation of viable pulmonary artery autografts through tissue engineering,xe2x80x9d Journal of Thoracic and Cardiovascular Surgery 115(3), 536 (1998); Niklason, L. E., J. Gao, W. M. Abbott, K. K. Hirschi, S. Houser, R. Marini and R. Langer, xe2x80x9cFunctional arteries grown in vitro,xe2x80x9d Science 284(5413), 489-493 (1999); Cao, Y., J. P. Vacanti, K. T. Paige, J. Upton and C. A. Vacanti, xe2x80x9cTransplantation of chondrocytes utilizing a polymer-cell construct to produce tissue-engineered cartilage in the shape of a human ear,xe2x80x9d Plastic and Reconstructive Surgery 100(2), 297 (1997); and my co-pending patent application entitled, xe2x80x9cPorous Composite Materials,xe2x80x9d U.S. Ser. No. 09/292,896, filed Apr. 27, 1999.
However, to engineer clinically useful tissues and organs is still a challenge. The understanding of the principles of scaffolding is far from satisfactory, and xe2x80x9cidealxe2x80x9d scaffolds are yet to be developed.
Pore size, porosity, and surface area (surface-to-volume ratio) are widely recognized as important parameters for a scaffold for tissue engineering. See Ishaug-Riley S. L., G. M. Crane-Kruger, M. J. Yaszemski and A. G. Mikos, xe2x80x9cThree-dimensional culture of rat calvarial osteoblasts in porous biodegradable polymers,xe2x80x9d Biomaterials 19(15), 1405 (1998). Other architectural features such as pore shape, pore wall morphology, and interconnectivity between pores of the scaffolding materials are also suggested to be important for cell seeding, migration, growth, mass transport, gene expression and new tissue formation in three dimensions.
In the body, tissues are organized into three-dimensional structures as functional organs and organ systems. Each tissue or organ has its specific characteristic architecture depending on its biological function. This architecture is also believed to provide appropriate channels for mass transport and spatial cellular organization. Mass transport includes signaling molecules, nutritional supplies, and metabolic waste removal. Spatial cellular organization determines cellxe2x80x94cell and cell-matrix interactions, and is critical to the normal tissue and organ function. To engineer a tissue or organ with a specific function, a matrix material (natural or synthetic) plays a critical role in allowing for the appropriate cell distribution and in guiding the tissue regeneration in three-dimensions. Therefore, to develop a scaffold for tissue engineering, the architectural design concerning the spatial cellular distribution, mass transport conditions, and tissue function is very important.
A few methods have been developed to produce porous scaffolds for tissue engineering. See Langer R., xe2x80x9cSelected advances in drug delivery and tissue engineering,xe2x80x9d Journal of Controlled Release, 62(1-2):7-11 (1999); Ma, P. X., T. Shin""oka, T. Zhou, D. Shum-Tim, J. Lien, J. P. Vacanti, J. Mayer and R. Langer, xe2x80x9cBiodegradable woven/nonwoven composite scaffolds for pulmonary artery engineering in a juvenile lamb model,xe2x80x9d Transactions of the Society for Biomaterials, 20:295 (1997); Yannas, I. V., xe2x80x9cApplications of ECM analogs in surgery,xe2x80x9d Journal of Cellular Biochemistry 56(2):188-191 (1994). The salt-leaching technique is a popular procedure to fabricate scaffolds from biodegradable polymers. See Thomson, R. C., A. G. Mikos, E. Beahm, J. C. Lemon, W. C. Satterfield, T. B. Aufdemorte and M. J. Miller, xe2x80x9cGuided tissue fabrication from periosteum using preformed biodegradable polymer scaffolds,xe2x80x9d Biomaterials 20(21):2007-2018 (1999). With this technique, the pore size can be controlled by the salt particle size, and the porosity can be controlled by the salt/polymer ratio. The pore shape is, however, limited to the cubic salt crystal shape. Textile technologies have been utilized to create woven (knit) and nonwoven fibrous scaffolds for tissue engineering applications. See Wintermantel E., J. Mayer, J. Blum, K. L. Eckert, P. Luscher and M. Mathey, xe2x80x9cTissue engineering scaffolds using superstructures,xe2x80x9d Biomaterials 17(2):83-91 (1996). The fiber diameter can be controlled at the micrometer level (typically around 15 microns), and the inter-fiber distance and porosity can also be manipulated to a certain extent with the processing variables. However, further lowering of the fiber diameter is limited with the textile technologies.
Thus, it is an object of the present invention to provide a method for forming a porous natural or synthetic material which advantageously has a designed and well-controlled macroporous architecture. It is a further object of the present invention to provide such a porous material which advantageously has substantially completely interconnected pores. Yet further, it is an object of the present invention to provide a method for forming an organized porous structure which advantageously avoids methods conventionally used to form random pore shapes and/or arrangements. It is yet a further object of the present invention to provide a method for forming, as well as the resultant porous material having a complex geometry which may advantageously incorporate random geometrically shaped materials into well-designed and controlled three-dimensional configurations.
The present invention addresses and solves the above-mentioned problems and meets the enumerated objects and advantages, as well as others not enumerated, by providing a method for forming a designed and well-controlled porous natural or synthetic material. The method comprises the step of casting a natural or synthetic composition onto a negative replica of a desired macroporous architecture of the porous material, thereby forming a body, the negative replica having been formed from a predetermined three-dimensional configuration of shaped porogen materials. The method further comprises the step of removing the porogen materials from the body, thereby forming the porous material having the desired macroporous architecture.